This invention relates to a method for the continuous preparation of liquid silicone rubber bases and in particular for the preparation of electrically conductive liquid silicone rubber bases.
Liquid silicone rubber compounds that cure by the hydrosilylation reaction to give colored or electrically conductive rubber are already well known. These compounds are in frequent use in the electrical, electronic, construction, and mechanical sectors. The electrically conductive liquid silicone rubber base that is the base ingredient of electrically conductive liquid silicone rubber compounds is often prepared by a batch method from a basic set of ingredients consisting of alkenyl-functional polyorganosiloxane, conductive filler, and optionally nonconductive inorganic filler.
In the batch scheme these precursors are introduced into a mixer such as a twin-blade kneader or dough mixer, a Henschel mixer, or planetary mixer and are mixed to homogeneity. A continuous production method is also available in which the precursors are continuously introduced into the twin-screw extruder described by Kobayashi, et al. in U.S. Pat. No. 4,649,005 and are continuously mixed and discharged. The batch production methodology suffers from such problems as a low productivity, high equipment costs, and poor ability to achieve the homogeneous dispersion of microfine conductive fillers such as carbon black. Moreover, achieving the homogeneous dispersion of a microfine conductive filler such as carbon black is difficult even with the preparative method using the twin-screw extruder.
Hamada, et al., in U.S. Pat. No. 5,599,102 disclose a continuous mixing apparatus for mixing a liquid with a powder, but does not disclose conditions by which a microfine conductive filler such as carbon black can be uniformly blended into a fluid silicone polymer.